Typical solar cells for direct conversion of sunlight to electricity have efficiencies in the range of 10-15 percent. This low efficiency, supplemented by high manufacturing costs, has limited their application. Basically, solar panel efficiencies have been limited for the following major reasons:
1. All photovoltaic materials have a particular characteristic energy gap which represents a minimum amount of energy an electron must receive to do useful work. The sun, however, emits a broad spectral distribution of photons, each having an intrinsic energy value given by E=hf; where h is Planck's constant and f is the frequency of the photon. Accordingly: PA1 2. A fraction of the incident photons are reflected from the surface of the material. PA1 3. A fraction of the incident photons miss the electrons in the photovoltaic material. PA1 4. A fraction of the incident photons are absorbed at distances greater from the junction than the so-called diffusion length. These photons generated non-useful carriers. PA1 5. There is a significant contact resistance in large solar cell arrays.
(a) photons which have energies below the energy gap of the photovoltaic material will either be reflected or absorbed and converted to wasted heat (which further reduces the efficiency of the solar cell); PA2 (b) photons which have an energy level equal to the energy gap convert nearly all their energy to useful work (electricity) providing that they are absorbed by the electrons; PA2 (c) photons which have energies greater than the energy gap, convert part of their energy to useful work and their excess energy (above the energy gap) is lost, as wasted heat.
Of the above, the first reason constitutes the major source of inefficiency, since the solar spectrum covers a broad range of frequencies with many photons below and above the energy gap of any typical photocell.
In spite of the fact that it has long been known that light can be easily separated into distinct frequency bands as with a prism, the inventor is aware of only one instance in which it has been suggested that the incident radiation be broken into separate energy bands and photovoltaic cells responsive to these energy bands disposed in the path of the appropriate radiation. See "Direct Generation of Electricity," K. H. Spring, Academic Press, 1965, pages 353, 354 and 355 and Proceedings of the 14th Annual Power Source Conference, U.S. Army Signal Research and Development Laboratory, Advanced Photovoltaic Devices, William R. Cherry, pages 37-42, 1960 (cited as footnote 21 in the Spring reference). Instead, most approaches to increase the output of photovoltaic cells have been directed to concentration of the incident radiation. When the incident radiation is concentrated, however, higher temperatures are produced in the photovoltaic cells, the efficiency of the cells is decreased and the need for cooling of the cells becomes necessary. According to this invention, I have developed a high-efficiency converter of solar energy directly to electricity at efficiencies higher than were heretofore available and which permits the use of concentrators without the attendant thermal disadvantages previously noted.
Accordingly, it is an object of this invention to provide a high efficiency converter of solar energy directly to electricity.
It is another object of this invention to provide for the direct conversion of solar energy to electricity in which the photo cells are not exposed to low energy IR radiation.